A fuel cell is composed of a solid electrolyte showing ion conductivity such as oxygen ion and proton, a porous air electrode (oxidation electrode: cathode) on one side surface of the solid electrolyte and a fuel electrode (reduction electrode: anode) on the other side surface of the solid electrolyte. Oxidation gas containing oxygen gas is supplied to the air electrode, and reduction gas containing hydrogen and hydrocarbon gas is supplied to the fuel electrode. These gases electrochemically react with each other through the solid electrolyte, thus generating electromotive force.
However, the electromotive force obtained by a fuel cell unit is as small as about 1.12 V, and a plurality of cells must be connected in series to obtain sufficient electromotive force for using them as a household power source or a power source for a car.
A solid oxide fuel cell (hereinafter referred to as SOFC) is one kind of fuel cells. The SOFC are divided broadly two categories. One is a cylindrical type in which electrodes and a solid electrolyte are covered around a cylinder and the other is a planar type in which a solid electrolyte and electrodes are formed to be planar.
The cylindrical type SOFC combined with the cylindrical cells shows difficulty in enlarging an area of a power generation portion (an area of a solid electrolyte) of the cell. Further a power density per unit volume is low when the cells are connected. Accordingly, how to increase the power density thereof has been a fundamental technical subject. On the other hand, the planar SOFC combined with the planar cells (cell plates) has an advantageous structure in increasing a power density per unit volume of a cell, and which is suitable for a power source of a moving body.
As the planar SOFC, as shown in FIGS. 1 and 2, the one has been known, which has a structure that cell plates 10A and 10B are stacked so that fuel electrodes 13 face to each other with interconnects 14b therebetween and air electrodes 12 face to each other with interconnects 14a therebetween (Japanese Patent Laid-Open publication H9-45355 (published in 1997)).
As shown in FIG. 2, this planar SOFC is composed of the cell plates 10A and 10B, each of which has planar solid electrolyte 11. Each of the cell plates 10A and 10B has the planar solid electrolyte 11 of a predetermined thickness, the porous air electrode 12 formed on one side surface of the solid electrolyte 11, and the porous fuel electrode 13 formed on the other side surface thereof. In this SOFC, to secure rigidity of the cell plate, one of the air electrode 12 and the fuel electrode 13 is formed to be thicker than other portions.
As shown in FIG. 1, the cell plates 10A and 10B adjacent to each other have vertically adverse arrangements as to the air electrode 12 and the fuel electrode 13. Specifically, as shown in FIG. 2, the uppermost cell plate 10A is arranged so that the air electrode 12 is allowed to face upward and the fuel electrode 13 is allowed to face downward. The second uppermost cell plate 10B is arranged so that the fuel electrode 12 is allowed to face upward and the air electrode 13 is allowed to face downward. Other cell plates are arranged in the same manner as the above. Consequently, the air electrodes 12 and the fuel electrodes 13 of the adjacent cell plates are allowed to face each other, respectively.
The conductive interconnect 14a for an electrical collection lead is interposed between the electrodes 12 and 12 which face each other as described above, and the conductive interconnect 14b for an electrical collection lead is interposed between the electrodes 13 and 13 which face each other as described above. Moreover, the interconnects 14a and 14b form gas flow paths, respectively. The interconnect 14a electrically connects the electrodes 12 and 12, and the interconnect 14b electrically connects the electrodes 13 and 13.
In this SOFC, a separator is unnecessary by adopting the above-described constitution. The problems of lowering of the electromotive force due to internal resistance of the separator and scaling-up of the fuel cell are solved.